The lag could be because all of a sudden many archives have been generated and due to network limitation have not yet been transferred to the standby location. The gap should reduce and eventually become 00 as it will be able to pace up during non peak hours. we often face this issue but its ok by the business.

Actually my concern are those database with smaller transactions. We have 50Mb size redologs, and some of the database took 1 day to be able to switch or fill it, hence the DR standby site is lagging 1day+. Do I need to command alter database switch log file every 30 minutes to force log apply?

I thought starting up the MPR0 process is gonna solve my non applied archive logs to STANDBY. But I noticed some logs are not updated, and there are missing archivelog at STANDBY like 3507 and 3508 and up to current, it was not auto transferred.